MEMS device manufacturing includes the construction of physical elements that are added to the traditional solid-state circuitry on chips known as integrated circuits (“ICs”). These physical elements can add significantly to the functional capability of the integrated circuit. For example, MEMS devices such as accelerometers, gyroscopes, gas density sensors, chemical or enzyme sensors, optical projection or redirection devices, micro-pumps, and pressure sensors have been fabricated. Usually, the physical motion or reaction of the MEMS device to external forces are measured through changes in capacitance, inductance, intrinsic resistance, etc. However, some MEMS devices cause physical action rather than measure physical forces such. Examples of such physical action include micro-pumping or mirror angle change.
The technology used to construct/manufacture MEMS devices has steadily evolved since its inception. At first, MEMS physical elements were constructed using such etching processes such as Reactive Ion Etching (“RIE”) and chemical bath etching because these processes were already available to IC fabs. One type of chemical bath etching used in early MEMS device construction was a hydrofluoric acid (“HF”) bath. The MEMS devices/features that could be successfully produced with these methods and equipment tended to be purely vertical in nature. When the MEMS features became largely horizontal and undercut, the features had to be large enough so that liquid bath processing could be used, and liquid withdrawn at the end of the process however, as MEMS undercut features became smaller and smaller, withdrawing or drying out the small capillaries or undercut features became more challenging. Liquid to wall adhesion or surface tension tended to cause collapse of the thin overhanging MEMS features. This problem is referred to as “stiction.”
In addition to stiction, HF bath etching faced additional disadvantages, namely an inability to achieve high etch selectivity of materials (e.g., the etching of oxide1 vs. oxide2). Other challenges included HF etching selectivity to metals that can be used to make contacts to the MEMS elements such as Aluminum, Copper, Gold, Silver, Titanium Nitride and other conductive materials.
In the early 1990s, an HF and alcohol etching process was developed as an alternative to the traditional HF water etch process to help eliminate some of the disadvantages in MEMS construction. The HF and alcohol etch process helped solve some of the purely chemical issues but did not help with the “stiction” problem, especially as features started to shrink toward the 10 or even 1 micron scale. While it is true that solid state circuit elements were being made in the micron scale at that time, the fact that MEMS are physical elements introduced additional processing hurdles that prohibited the construction on micron scale. Thu, a better process was needed that could etch small pathways without creating “stiction” and without attacking adjacent materials.
In the middle 1990s, a process called Super-Critical-Carbon Dioxide (“CO2”) drying was developed. This process, which operates at about 2000 pounds per square inch (“PSI”) and at about 30 degrees Celsius (° C.) using CO2 gas, is able to remove liquids in small capillaries. While this process was marginally successful, it was also cumbersome because it required a separate piece of equipment that allowed the MEMS chips to partially dry during transport to the supercritical CO2 equipment. This partial drying during transport caused stiction.
A gas phase, atmospheric pressure, process was also developed at this time using HF and alcohol to eliminate some of the problems with wet etching. While this gas phase HF/alcohol etching process was able to etch in small places, it often left liquid residues based on the byproducts of the etch since the byproduct vapor pressure was lower than the processing pressure.
Primaxx, Inc. then developed a reduced pressure HF/alcohol etch process to overcome the deficiencies of the atmospheric HF/alcohol process. This process is described in U.S. Pat. No. 5,439,553, Grant et al., the teachings of which are hereby incorporated by reference in its entirety. The reduced pressure HF/alcohol etch process depends heavily on the vapor pressures and partial pressures of the constituent gases. Several advantages were realized by the reduced pressure HF/alcohol etch process, including the elimination of “stiction” and the minimization of attack on companion materials such as metals and nitrides. The reduced pressure HF/alcohol process was developed in a single wafer reactor, and was integrated into a cluster tool using, for example a Brooks Automation robotic handler. However, in order to control “stiction” and etch selectivity, the application of the reduced pressure HF/alcohol etch process proved to be a fairly lengthy process, often taking 20-30 minutes (or longer) to complete release etching of the MEMS device. In order to become viable in production it was thought that the process needed to be reduced.
The problem in implementation however came in the form of wafer-to-wafer uniformity. The etch uniformity throughout the wafer needed to stay well ahead of the 10% uniformity limit. This proved to be difficult due to the construction requirements of a cluster tool. The wafer loading and unloading requirements placed a burden on fluid dynamics and other process control design criteria.
In order to solve the throughput problem, it was thought that a multiple wafer reactor could be constructed, but in practice, the top and bottom wafers in a stack of wafers often etched differently than the rest of the batch. Also, the “slit valve” feature where wafers were loaded created a slight swirl or disturbance to the gas flow in the reactor. The temperature control of a stainless steel or nickel reactor (required due to material compatibility with HF gas) was found to be an important factor. Variations in temperature were found to result in variations in etch uniformly across the wafers. These structural problems were addressed in Primaxx's recent design of a batch processing chamber for MEMS construction, which is the subject of a co-pending U.S. patent application Ser. No. 10/991,554, filed Nov. 18, 2004. However, deficiencies in the available etching process, including the reduced pressure HF/alcohol etch, still presented substantial obstacles with respect to yield.
As discussed above, reduced pressure gas phase HF/alcohol etching has proven to be very useful in the production of MEMS devices but has some drawbacks. Some of the earliest reports of HF/alcohol reduced pressure SiO2 etching are Reduced Pressure Etching of Thermal Oxides in Anhydrous HF/Alcoholic Gas Mixtures, by Kevin Torek and Jersey Ruzyllo; Modeling and Characterization of Gas-Phase Etching of Thermal Oxide and TEOS Oxide Using Anhydrous HF and Ch3OH, by Chun Su Lee et al; and Fabrication of MEMS devices by using anhydrous HF gas-phase etching with alcoholic vapor, by Won Ick Jang, Chang Auck Choi et al. In these papers and related patent work, specific alcohols were determined to provide good results using various criterion. In the Torek paper, methanol was selected over ethanol or isopropanol due to its superior residue performance while etching oxides of silicon prior to gate oxidation and other critical applications. In the Choi work, the amounts of reactants (HF and methanol) on the surface were shown to be proportional to the partial pressures and had an influence on oxide to oxide type selectivity. In the Jang work, methanol and isopropanol were cited as excellent catalysts for MEMS silicon oxide release etching due to superior etch control with no “stiction” of the MEMS released beams.